It's what we Brits do to each other in class every first day of a new month. We give quick nips and medium punches to our chums' upper arms as a means of showing each other our love and affection for them.
Adding "No returns" isn't a fundamental requirement, but the recipients are allowed to give the same back without it being stated - and they're quite within their rights to add several newtons of force.
And, you wouldn't want that. Kind of like not knocking John Wayne off his feet when you had the chance: